This invention relates in general to container closures, and more particularly to a mold assembly and method for forming container closures that have multiple sealing surfaces and a tamper-evident ring.
Various tamper-evident closures have been proposed wherein a threaded cap has a skirt portion with internal threads for connecting the cap to the threaded neck of a container. A security ring is typically formed during molding of the cap and frangible connections are formed after molding by cutting or slitting openings into the closure at the juncture of the security ring and skirt portion. The security ring may be provided with a plurality of circumferentially spaced, inwardly and upwardly extending tabs or fingers integrally connected to the ring. The outer free ends of the fingers are adapted to engage beneath a shoulder or ledge on the container neck below the threaded portion. In this manner, the cap is prevented from being removed from the container until twisted a sufficient distance to break the frangible connection and thereby separate the cap from the ring.
Closures of the above-mentioned type are typically manufactured with medium or high density polyethylene materials through die casting or thermoforming. The materials must be sufficiently resilient and flexible to enable extraction of the male die member from the cavity of the closure despite the presence of the inwardly and upwardly extending fingers. Since the closures are anchored to the container mainly by engagement of the fingers with the shoulder or ledge, the required flexibility and resilience of the closure to enable separation from the die may also enable the closure to be relatively easily removed from the container without breaking the frangible connection between the skirt and the security ring by slipping the closure off the container in a direction parallel with the axis of the container neck. This is particularly disadvantageous since there is no clear indication that the container has been tampered with.
In recognition of this problem, UK Patent Application No. GB 2 039 817 proposes die molding a closure out of relatively rigid plastics material, such as polypropylene. However, the die-molding process itself is relatively time consuming and therefore contributes to an increase in the cost of manufacturing the closure. In addition, the formation of frangible bridges requires a further processing step wherein openings between the bridges are cut or slit at the intersection of the security ring and skirt portion of the closure. The extra processing step requires extra equipment, time, and energy and thereby contributes to the overall cost of the closure.
Other closure designs depend on adding a different material on the bottom surface of the closure top wall to maintain sealing against leakage. The addition of any material to the cap requires more steps in manufacturing and assembly, and also contributes to an increase in closure manufacturing costs. Thus, it is desirous to produce a closure that is of single piece construction.
As desirable as the one-piece plastic closure may be, there are a number of problems associated with its use. For example, it is difficult to maintain a seal between the closure and container over an extended period of time under various conditions of packing, storing and handling. Once the closure is firmly engaged on the container, whether by threads or some other means, various parts of the closure are under stress because of forces required to effect a seal. These stresses tend to cause cold flow or creep of the closure material and may cause a breach in the seal between the closure and container. This tendency is augmented when the closure and container are subject to internal pressure from carbonated beverages. A plastic material""s tendency to creep is further affected by elevated temperatures, which may result in a breach of the seal and consequential leakage and/or spoilage of the contents.
Even when the closure is sufficient to retain the beverage in a carbonated state, removal of the closure for dispersing the beverage causes seal degradation to the point where the closure cannot retain its prior sealing ability. Consequently, the beverage will lose much of its carbonation in a relatively short period of time and is often discarded before being completely consumed.
In addition to the above drawbacks, the internal threads in prior art closures are typically symmetrical in cross section. Although this construction may facilitate attachment of the closure to the container, the internal threads of the closure may not properly seal against the external threads of the container. Moreover, the molding process for closures with threads that are symmetrical in cross section typically requires the core mold to be rotated or unscrewed from the interior of the closure after closure formation. This is a time consuming process and is much less efficient than if the core mold were to be withdrawn from the formed closure without rotation.
According to the invention, a mold assembly for a container closure is disclosed. The container closure has an end wall and a skirt extending from the end wall. The skirt includes an inner skirt surface with an internal, asymmetrical helical thread formed thereon for engaging an external helical thread of a container. The internal, asymmetrical helical thread has a ramped surface. A security ring is frangibly connected to the skirt, and a shoulder extends between the security ring and the skirt. The mold assembly comprises first and second mold portions cooperating to define, in a closed position, a closure cavity. The second mold portion has a stripper ring adapted for engaging the shoulder of the closure, and a threaded core having an external, asymmetrical helical thread defining the internal, asymmetrical helical thread of the closure. The external, asymmetrical helical thread has a ramped surface defining the ramped surface of the internal, asymmetrical helical thread. With this arrangement, relative linear movement between the stripper ring and the threaded core causes withdrawal of the threaded core from the closure in a linear direction, with the ramped surfaces of the internal and external, asymmetrical helical threads cooperating to facilitate the withdrawal.
Further according to the invention, a mold assembly for a container closure having an end wall, an annular seal and a skirt extending from the end wall and a frangible security ring extending from the skirt, comprises first and second mold portions cooperating to define, in a closed position, a closure cavity. The second mold portion has a threaded core and an inner core located in the threaded core. The inner core and threaded core cooperate to define an annular seal on the end wall of the closure. Relative movement between the inner core and threaded core creates a first clearance space into which the annular seal can deform during ejection of the closure from the mold assembly. An outer core surrounds the threaded core. The outer core and threaded core cooperate to define at least an inwardly extending flap portion of the security ring. Relative movement between the outer core and threaded core creates a second clearance space into which the flap portion of the security ring can deform outwardly during ejection of the closure from the mold assembly.
These and other features and advantages of the invention will become apparent upon reference to the following detailed description and the accompanying drawings.